drm/amd/display: Soft reset DMUIF during DMUB reset

[Why]
We need to ensure that the DMUIF in MMHUBBUB is also in reset so we
aren't generating requests while the DMCUB is in reset.

[How]
Set DMUIF_SOFT_RESET=1 on reset and DMUIF_SOFT_RESET=0 on reset
release.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>